I have lots of little clothes with Mickey on them, which my kids wore once upon a time. I’ve found that the best size for Duffy is 12 months.
I simply cut through the front and back of the t-shirt , turn it inside out, and sew up and around the part I cut:
Then I turn it inside out, and cut a thin slit in the back where Duffy’s tail will be.
This is the finished result: